High temperature, lightweight aerospace structural composites are required for high Mach number airframes and next generation propulsion systems requiring high strength-to-weight and stiffness-to-weight ratios. Titanium alloys and titanium aluminides are attractive materials for this application. Although considerable effort has gone into alloy and metal matrix composite development, the applicability of these advanced structural materials at elevated temperatures will ultimately depend on their resistance to the oxidizing gases that are an integral part of their operating environment. This is due to the sensitivity of the mechanical properties of titanium alloys to oxygen embrittlement. Because of the high solubility (i.e., 30 at. %) and diffusivity of oxygen in titanium at elevated temperatures, exposure to an oxidizing environment leads to the growth of a non-protective oxide film that becomes a source for oxygen dissolution into the alloy substrate.
Significant reductions in ductility, fatigue and creep properties of titanium alloys and titanium aluminides may occur since oxygen concentrations exceeding 1 at. % in the alloy matrix are sufficient to cause significant loss in ductility and fracture toughness. It is thus expected that under operating conditions cracks will nucleate and propagate from such an embrittled surface layer. The application of overlay coatings to protect against oxidation may also lead to localized embrittlement of the substrate and subsequent loss of mechanical properties due to either interdiffusion and/or brittle intermetallic compound formation at the coating-substrate interface. In addition, thermal and/or mechanical strains (i.e., fatigue) that may occur during usage can lead to cracking or spallation of the coating with subsequent loss of protectiveness. Surface modifications that provide oxidation protection over a wide range of operating conditions and which do not degrade substrate mechanical properties are needed to fulfill the high temperature structural potential of titanium alloys and titanium aluminides.
In the prior art successful surface modifications were developed for vanadium-base alloys for use in oxidizing gases in fusion reactor blanket applications. It was shown that surface alloying of vanadium base-alloys with chromium was effective in preventing both oxygen absorption and oxygen diffusion into the alloy substrate in moist He environments. This principle was applied to surface protection of several commercial titanium alloys and .alpha..sub.2 titanium aluminides in which aluminum was diffused into the alloy via a vapor transport process and led to a significant improvement in cyclic air oxidation resistance up to 815.C. However, it was also shown that this process may also lead to some reduction in tensile properties of the substrate.